Heterogeneity is a pervasive feature of cancer, central to progression and therapy failure. Both genetic and nongenetic factors, cell-cell and cell-microenvironment interactions contribute to tumor cell phenotypic variation. Thus, heterogeneity is a complex multiscale problem difficult to study by reductionist approaches but well suited to systems thinking. A mechanistic, system-level understanding of heterogeneity would spawn fundamental advances in cancer treatment strategies. Major challenges include definition of relevant tumor cell phenotypes, phenotype dynamics emergence from single-cell behavior and interactions, and effective targeting strategies. In our Center, we will use systems biology approaches to tackle these challenges, focusing on small cell lung cancer (SCLC), in which the impact of intratumor heterogeneity is particularly compelling. Thus, SCLC tumors, while histo-pathologically homogeneous with classic ?small blue round cell? morphology, are comprised of phenotypic subpopulations (e.g., tumor-propagating cells; Hes1+ cells; CD44+ cells) that cooperate to form a tumor ecosystem adaptive to drug treatment. In SCLC genetically engineered mouse models (GEMMs) and patient tumors, accumulated genetic alterations (e.g., MYC amplification, NOTCH mutations) may bias phenotypic compositions and consequent drug sensitivity, underscoring the combined role of genetic and nongenetic sources. The overall goal of our proposed Center is a system-level understanding of the impact of SCLC phenotype heterogeneity in drug evasion, that will open avenues to novel treatment strategies. In two highly integrated Projects, we will combine experimentation with mathematical modeling to generate a comprehensive blueprint of SCLC phenotypic space and identify complex phenotype dynamics underlying treatment resistance. Project 1 will use high-dimensional cytometry and transcriptomics data to define human and GEMM core SCLC phenotypes, their susceptibility to genomic alteration bias, and their drug response plasticity. Project 2 will study SCLC phenotype dynamics initiated by cell-cell and secreted factors or exosomes in SCLC tumor ecosystems, using models of cell population dynamics driving tumor aggressiveness. Projects will be supported by a Single-Cell Biology and Data Analysis Shared Resource equipped with a vast palette of state-of-the-art single-cell technologies, including mass cytometry, scRNA-Seq and multidimensional data analysis. The Center will be located at Vanderbilt University and a satellite site at Stanford contributing highly-regarded SCLC GEMMs. Experimental systems will encompass human and GEMM SCLC cell lines and tumors. Our Center investigators have a track record of co-authored publications in SCLC and/or systems biology. The Administrative Core will provide leadership and communication across the Center while the Outreach Core will promote collaborations and disseminate knowledge and tools on SCLC and cancer heterogeneity. We expect that iteration between experiments and modeling will bring about a system-level understanding of SCLC tumor heterogeneity with lessons generally applicable to any cancer type.